Cleaning is a mutually beneficial relationship, where the cleaner removes and consumes ectoparasites from its clients. Among fishes, the cleaning behavior has evolved several times as a juvenile or facultative feeding strategy, and is often lost in adults. However, a minority of taxa clean almost exclusively across their ontogeny (obligate cleaners). The largest radiation of obligate cleaners are the Caribbean neon gobies (Elacatinus), that evolved with several congeneric non-cleaners, and in parallel with a closely related lineage of facultative cleaners (Tigrigobius). We suspect that obligate cleaning is a specialized trophic ecology tied to an equally specialized feeding morphology that facilitates improved cleaning performance. We coupled micro-CT scanning with the clearing and staining of museum specimens to compare several functional predictors of feeding performance between cleaner gobies and non-cleaners in a phylogenetic context. We also used geometric morphometrics to compare relative changes in head shape and mouth orientation. Obligate cleaning evolved once, and is characterized by a specialized scraping dentition, stouter cranial features, and a more subterminal mouth position relative to non-cleaners. Meanwhile, facultative cleaning evolved at least 2-3 times, and while some species display similar tooth morphologies as the obligate cleaners, they share more in common with non-cleaners. Obligate cleaners also exhibit fewer sexual dimorphic differences, suggesting that the demand for an efficient cleaning morphology trumps sexual selection.

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